<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form name="loginForm" method="post" action="./post">
        <div>
            <label for="phoneInput"></label>
            <input type="text" name="phone" id="phoneInput" 
            placeholder="请输入手机" required pattern="/^[0-9]{11}$/">
        </div>
        <div>
            <button type="submit">提交<tton>
        </div>
    </form>

    <script>
        const oForm=document.forms["loginForm"];
        oForm.addEventListener('submit',function(event){
            event.preventDefault();
            const phone=oForm["phone"].value.trim();
            // 为空
            // required pattern  html5才有的
            if(!phone.length){
                console.log('手机号不能为空');
                return;
            }
            if (!/^[0-9]{11}$/.test(phone)) {
                alert('手机号不能为空')
                return
            }
            this.submit();
        })
    </script>
</body>
</html>